Unexamined Japanese Utility Model Publication No. 6-41921 and Unexamined Japanese Patent Publication No. 7-290156 disclose a known conventional mandrel for bending comprising a plurality of plugs coupled in series onto an end of a shank. As can be seen in FIG. 4, the outer peripheral surface of each plug 50 is formed to be spherical and a connection shaft 52 is inserted through the center of the plug 50. The connection shaft 52 is provided with a spherical part 54 at one end and a spherical depression 56 at the other end.
The spherical part 54 of the connection shaft 52 is fitted into the spherical depression 56 of another connection shaft 52, which thus constitutes an adjustable joint. In this manner, a plurality of plugs 50 are connected in series. Moreover, the shank 58 is provided with a spherical depression 60 which is identical to the spherical depression 56. The spherical part 54 of the terminal connection shaft 52 is fitted into the depression 60 so that all the plugs 50 are connected to the shank 58.
When bending is performed, the plugs 50 are inserted into a pipe 62 to be arranged at the bending part. While the bending is performed, the spherical parts 54 rotate within the spherical depressions 56, 60 and the plugs 50 are waggled along the bending direction. As a result, spaces between the respective plugs 50 are narrowed and the outer peripheral surfaces of the plugs 50 are abutted to the inner wall of the pipe 62 in a concentrated manner on the inside of the bend.
Thus, formation of irregularities, such as wrinkles, on the inside of the bend is inhibited.